1. Field of the Invention
The present invention relates to a high-pressure discharge lamp, which exhibits little blackening.
2. Description of the Related Art
In general, a high-pressure discharge lamp is a light source, which provides a pair of electrodes inside a translucent quartz arc tube filled with a noble gas for starting, and mercury or another metallic halogen compound, and which is designed so that an arc discharge is generated by applying voltage to both electrodes and creating a current. This arc discharge illuminates the filling substance, enabling the high-pressure discharge lamp to be widely used as ordinary lighting, or as lighting for such equipment as an overhead projector (OHP).
A metallic halogen compound-filled metal halide lamp features especially high efficiency and high color rendering capabilities. For this reason, it has recently come into widespread use in combination with a reflecting mirror in liquid crystal projectors and other such image projecting devices. And for this type of metal halide lamp, as disclosed in Japanese Patent Laid-Open Publication No. 3-219546, for example, an iodide of neodymium (Nd), dysprosium (Dy) and cesium (Cs) is generally used as the metallic halogen compound contained in the arc tube.
A lamp containing an iodide of neodymium (Nd), dysprosium (Dy) and cesium (Cs) (hereafter referred to as a Dyxe2x80x94Ndxe2x80x94Csxe2x80x94I lamp) features outstanding luminous efficacy and color rendering, color temperature, but due to the strong reaction between neodymium (Nd) and the quartz in the arc tube, devitrification of the arc tube occurs during early life. Because this type of devitrification decreases luminous flux, reduces luminance and causes light to diffuse, it brings about uneven illuminance and reduced brightness in a liquid crystal projector screen. That is, when a Dyxe2x80x94Ndxe2x80x94Csxe2x80x94I lamp is used as the light source in a liquid crystal projector, good light generation characteristics are achieved, but the drawback is short lamp life.
To counter this, as is disclosed in Japanese Patent Laid-Open Publication No. 2-186552, a method for filling the arc tube with lutetium (Lu), which does not readily react with quartz, has already been reported. That is, devitrification can be decreased and a metal halide lamp with good light generating characteristics can be achieved by filling an arc tube with mercury and noble gas, and between 2xc3x9710xe2x88x927 mol/cc and 2xc3x9710xe2x88x925 mol/cc of lutetium (Lu) together with halogen.
Recently, because metal halide lamps used in liquid crystal projectors and other image projection devices are being combined with optical systems, which utilize liquid crystals, it is desirable to enhance optical efficiency by further shortening the arc length (distance between electrodes).
However, when the arc length is shortened, the thermal burden on the electrodes increases, giving rise to early blackening of the arc tube, and causing a dramatic drop in the luminous flux maintenance factor. That is, a lamp with a short arc length is disadvantageous in that the arc tube blackens and luminous flux decreases even sooner than with the arc tube devitrification phenomenon, even when filled with a substance that does not readily react with quartz.
An object of the present invention is to solve for this problem by providing a high-pressure discharge lamp that exhibits little blackening.
To achieve the above-mentioned object, the present invention is a high-pressure discharge lamp, which comprises a pair of electrodes that are separated from one another by a predetermined distance, and which is lighted by a reverse polarity power source, wherein this high-pressure discharge lamp is designed to satisfy a relationship whereby                     1.5        ≤                  (                      I                          π              ·                              r                2                                              )                ≤        9                            (        1        )            
when the radius at the tip of each electrode is r(mm), the lamp current at steady discharge is I (amperes), and the ratio of the circumference of the circle to its diameter is xcfx80.